A 28-year-old female with a history of recurrent skin and soft tissue infections presents with a 4-day history of fever, malaise, and a rapidly enlarging, painful boil on her left thigh with surrounding erythema and warmth. Pus is aspirated and sent for investigation. Gram stain shows gram-positive cocci in clusters. Which investigation is most appropriate to determine if this isolate is methicillin-resistant and guide empiric antibiotic therapy?
A. Coagulase test on the pus sample
B. Vancomycin E-test on the isolate
C. Mannitol fermentation test on blood agar
D. Oxacillin disk diffusion (Kirby-Bauer) or cefoxitin disk diffusion test
Explanation
Methicillin Resistance Detection in S. aureus
Why Oxacillin/Cefoxitin Disk Diffusion is Correct
Key Point
Oxacillin disk diffusion (or cefoxitin disk diffusion as a surrogate) is the CLSI-recommended screening method for detecting methicillin resistance (MRSA) in Staphylococcus aureus isolates and is the most practical, cost-effective investigation for guiding empiric therapy.
High-YieldNEET PG
Methicillin resistance detection is CRITICAL because:
MRSA requires vancomycin, linezolid, or daptomycin
MSSA responds to beta-lactams (nafcillin, oxacillin, cephalosporins)
Empiric therapy selection depends entirely on MRSA status
Interpretation of Disk Diffusion Results
Table
Test
Disk Used
Resistant Zone
Interpretation
Oxacillin disk diffusion
1 μg oxacillin
≤10 mm
MRSA (methicillin-resistant)
Cefoxitin disk diffusion
30 μg cefoxitin
≤17 mm
MRSA (more sensitive than oxacillin)
Vancomycin E-test
Vancomycin strip
MIC ≥2 μg/mL
Vancomycin-intermediate (rare)
Clinical Pearl
Cefoxitin disk diffusion is now preferred over oxacillin because it better predicts mecA gene presence and is more sensitive for detecting borderline oxacillin-resistant strains (BORSA).
